A double ‘relative risk’ (RRx2) test used by courts and government agencies to determine if a cancer is occupational ‘is useless’, top US scientists have said. The RRx2 system  requiring a condition to be twice as common in the affected group than in the general population is used by lawyers and state compensation agencies to set a cut off below which compensation will not be paid. But Richard Clapp and David Ozonoff, writing in 2004 in the American Journal of Law and Medicine, note: “To an epidemiologist using generally accepted methods of epidemiological analysis, however, a RR of 2.0 or more is not necessary in order to show that a causal link is ‘more likely than not’ present in the study population. In our experience as epidemiologists who participate in the legal process as experts, some attorneys maintain and some courts believe that a RR of 2.0 is needed before one can conclude from an epidemiological study that the outcome was ‘more likely than not’ due to the exposure. The arithmetic basis of this proposition would seem quite transparent, but like many things in the subtle and complex science, there are sound and accepted reasons why this argument is not valid. The reasons are both technical and ethical.” They note that the relative risks across a “usually hetrogenous” study group could mask much higher and genuine occupational risks in someone without few or none of other risk factors present in the group, leading to “a serious under-estimate of the effects of his or her exposure.” They add: “Without a specification of the underlying causation model, which in almost all cases is insufficiently known to allow an accurate calculation, or even any calculation, of the fraction of cases due to exposure, the doubling of the RR… is useless as a criterion for evidentiary admissibility.”

Clapp RW, Ozonoff D (2004). Environment and health: vital intersection or contested territory?  American Journal of Law and Medicine, volume 30(2-3), pages 189-215.